1. Field of the Invention
This invention relates in general to oil and gas wellheads, and in particular to means for suspending multiple casing strings in a well.
2. Description of the Prior Art
In an oil and gas well, a wellhead or wellhead housing will be located at the ground surface. The wellhead housing is a tubular member having an axial bore. A conical load shoulder is located in the bore.
If the well is a deep well, normally there will be more than one string of casing extending into the well. The first string of casing is connected to a casing hanger which lands on the load shoulder in the wellhead housing. Cement is pumped down the string of casing and back up the annulus surrounding the casing to cement the casing in the well. The return of fluid from the casing passes through bypass or return flow passages on the side of the casing hanger. After the cementing is completed, a packoff is sealed between the casing hanger and the wellhead housing.
The well will then be drilled to a greater depth at a smaller diameter. The second string of casing may also extend completely to the wellhead housing and be supported by a casing hanger. In one technique, the second casing hanger is stacked on top of the first or lower casing hanger. The weight of the smaller diameter casing is transmitted through the lower casing hanger to the wellhead housing load shoulder. The second string of casing will also be cemented in place. A packoff will be sealed between the second casing hanger and the wellhead housing bore. In some wells, a third string of casing will also be run and supported by a casing hanger stacked on top of the first two casing hangers.
The load or landing shoulder in the wellhead housing must be very strong. It must be able to support the weight of the multiple strings of casing. Also, high pressure in the wellhead acts on the casing hangers, exerting a high force on the load shoulder.
For this reason, if the pressures will be very high and the casing weight is very high, mechanisms are employed to transfer the weight separately from each casing hanger directly to the wellhead, rather than to the single load shoulder. While this is workable, it results in considerable extra expense. The load mechanisms are expensive. Also, these devices require more length for the wellhead housing, therefore increasing the cost for this item.